The present invention relates to a method for reducing the build-up of slime and/or film in a plant that circulates water. The invention is also based on a system for carrying out this method, as well as the various applications of this method for preventing the build-up of slime and/or film in a wide variety of industrial processes utilizing water circulation.
Water circulation, in particular the closed circulation of white water in paper machines, such as a twin-white former paper machine, and/or the circulation of cooling water, involves the problem that slime and/or film forms and builds up on the solid surfaces that are in contact with the liquid phase involved in the circulating process (cf. "Vortrage anlablich des 5. PTS-WAF-Lehrgangs, Fortbildungskurs zur Wasser- und Abwasseranalytik, Kontrolle des Einsatzes von Mikrobioziden in der Papiererzeugung" ("Lectures in the Fifth PTS-WAF Course, Further Education on Water and Waste Water Analysis, Control of the Use of Microbiocides in Paper Production"), organized by the "Papiertechnische Stiftung," Munich, and Hans-Dietrich Held "Kuhlwasser" ("Cooling Water", Vulkan-Verlag Dr. W. Claussen, Essen, 1977, pp. 70-73)).
In the closed circulation of the white water, a process which is increasingly being used for paper machines, microorganisms find good growth conditions due to the high organic and inorganic nutrients present in the white water, as well as the favorable environmental conditions produced in the closed circulation systems, i.e. high temperatures, pH values close to neutral, and ample supplies of oxygen. Since many of the microorganisms are not present in the circulating water as free organisms but settle onto the fibrous, and/or fine materials present in the circulating system, as well as the surfaces of the machine parts, such as pipes, vessels, and pumps, an undesirable build-up of slime and/or film results. When the slime and/or film is detached from these surfaces, it often leads to the formation of lumps in the circulating material which may produce damage to the circulating end product such as the production of holes in the paper web produced by circulating water paper machines. This weakens the paper web, causes it to tear, thereby resulting in machine shutdowns. In order to prevent this build-up of slime and film, it is known to add biocides, lignosulfonates, or enzymes to the white water.
The use of biocides suppresses the growth of the microorganisms in the circulating water, as well as partly damages the organisms structure. However, the use of biocides is increasingly being criticized. When a greater amount of biocides is being used, it results in an increasing amount of damage to the environment to which the white water is released when the circulation process has been completed and the waste water has been emptied. Since the microorganisms tend to become resistant to biocides, it is also necessary to change the biocide substances frequently and/or to increase their amount. This means a increase in pollution or considerable costs, e.g. the necessity of utilizing a clarification plant or a drainage ditch for preventing such pollution damage.
Furthermore, in addition to the use of biocides to combat the build-up of microbial slime and/or film, lignosulfonates are also used as so-called "complexers" which prevent the microorganisms from taking in food under certain conditions. Sometimes a biocide may also be used with the lignosulfonates, but a much smaller amount of the biocide is necessary than if the biocide is used alone (cf. German Patent No. 34 47 686). Thus, even with the use of lignosulfonates the problems of biocides continue to exist but in milder forms.
In addition, enzymes may also be added to the circulation water in order to convert the high molecular polymers, which promote the build-up of slime and film, into low molecular products. This method is environmentally safe but has not proved to be very useful in large scale applications, presumably because it only allows for a brief reduction of viscosity while the hydrolysis or other low molecular products formed by the enzymes may even be the preferred food for the slime forming microorganisms.
Cooling water circulation involves the problem that leaks, or points of contact within the circulating product stream such as the condensation of gases in the cooling circulation which arise during vaporization processes in the circulating product stream, cause organic impurities to frequently pass into the circulating water. This again leads to the growth of microorganisms. Together with the organic and inorganic impurities, these microorganisms form deposits on the surfaces of the heat exchangers, thereby drastically reducing the efficiency of heat transmission. This necessitates the frequent and expensive cleaning of the heat exchangers in order to maintain the heat removal efficiency needed for the particular process. In this case, biocides are also used to reduce the build-up of slime and film on the heat exchanger surface.
The present invention is therefore based on the problem of finding an environmentally safe method of reducing the build-up of slime and film in water circulation systems, thereby reducing the shut-down periods of the processing plant.
This problem is solved by the methods stated below whose characterizing feature is that additional specific non-sessile microorganisms are added in single form to the circulating water. Surprisingly enough, when the inventive method is carried out and the additional microorganisms are selectively added to the circulating water in accordance with its organic load, there is a clear reduction in the build-up of slime and/or film on the solid surfaces and in the liquid phase.